Thermoptic materials exhibit changes in physical properties such as electrical conductivity, absorption, reflectance and refractive index as a result of thermodynamic state changes between the semiconductor and metal state. Thermoptic materials, such as certain oxides of vanadium and titanium have relatively high transmittance in the semiconductor state and high reflectance in the metal state over an extended region of the electromagnetic spectrum.
The changes in physical properties that are associated with the thermodynamic state change are large and occur over a small range of temperature; i.e., there is some transition temperature below which the material is a semiconductor and above which it is a metal. In the immediate region of this transition temperature, the optical properties, such as transmittance or reflectance are very temperature sensitive. Moreover, these physical properties exhibit a hysteretic behavior around the transition temperature, especially when the material is in thin film form. That is, the transition temperature is higher when increasing temperature from a lower value than the transition temperature which occurs upon cooling from a higher temperature.
It is now well-known that images can be recorded in films of thermoptic materials at extremely high speeds by heating a region of the film with a laser or electron beam. As long as the ambient temperature of the film is maintained at a suitable recording temperature inside the recognized hysteresis loop, these images will be stored in the film. Normally, such images are erased by allowing the entire film to cool from its recording temperature to some temperature below the loop.
When an image has been recorded (stored) in a film of thermoptic material, that film can be used as an efficient, two-dimensional spatial modulator of infrared radiation. By the use of two such thermoptic films as modulators in a system of infrared projection optics that allows alternate modulators to project successive frames of dynamic infrared imagery, it is possible to have a cinematic infrared scene projection system. The image storage property of the thermoptic films makes it possible for the projected dynamic scene data to be flicker-free, a feature which is important to but not heretofore available in the testing of infrared systems.